The long term goal of this program are to understand the structural basis for signal transduction in the epidermal growth factor (EGF) family of peptide hormones and to develop pharmacological tools that will aid in studying questions of cancer biology related EGF-mediated transduction. Of particular interest is the signaling pathway controlled by the ErbB3 and ErbB4 receptors and their selective activation by certain members of the neuregulin subfamily of hormones. Neuregulin2beta (NRG2beta) is a potent ligand for ErbB4 while Neuregulin2beta (NRG2beta) is a potent ligand for ErbB4 while Neuregulin2alpha (NRG2alpha) is not. The main hypothesis is that five unique amino acid residues in the carboxyl terminus of NRG2beta are responsible for selectively binding ErbB4. This hypothesis will be tested through the following specific aims: (1) Site-directed mutagenesis of the five amino acid residues in the carboxyl terminus of NRG2alpha to the corresponding residues in NRG2beta; and (2) evaluation of activity of the NRG2alpha mutants for ErbB4 activation (cell-based assays) and binding (in vitro assay) to determine the residues sufficient for ErbB4 activation; (3) Molecular modeling of NRG2alpha mutants to determine structural determinants potentially responsible for ErbB4 recognition and activation; (4) Synthesis of peptides having the C-terminal domain (at least residues 33-51) containing the key Cys33-Cys 42 disulfide linkage) of NRG2alpha and NRG2beta and their evaluation as potential agonists or antagonists in ErbB4 activation.